Earthquakes in the Central Virginia Seismic Zone

Since at least 1774, people in central Virginia have felt small earthquakes
and suffered damage from infrequent larger ones. The largest damaging
earthquake (magnitude 4.8) in the seismic zone occurred in 1875. Smaller
earthquakes that cause little or no damage are felt each year or two.

Earthquakes in the central and eastern U.S., although less frequent than
in the western U.S., are typically felt over a much broader region. East of
the Rockies, an earthquake can be felt over an area as much as ten
times larger than a similar magnitude earthquake on the west coast. A
magnitude 4.0 eastern U.S. earthquake typically can be felt at many
places as far as 100 km (60 mi) from where it occurred, and it infrequently
causes damage near its source. A magnitude 5.5 eastern U.S. earthquake
usually can be felt as far as 500 km (300 mi) from where it occurred,
and sometimes causes damage as far away as 40 km (25 mi).

Faults

Earthquakes everywhere occur on faults within bedrock, usually miles
deep. Most bedrock beneath central Virginia was assembled as continents
collided to form a supercontinent about 500-300 million years ago,
raising the Appalachian Mountains. Most of the rest of the bedrock formed
when the supercontinent rifted apart about 200 million years ago to
form what are now the northeastern U.S., the Atlantic Ocean, and Europe.

At well-studied plate boundaries like the San Andreas fault system in California,
often scientists can determine the name of the specific fault that is
responsible for an earthquake. In contrast, east of the Rocky Mountains
this is rarely the case. The Central Virginia seismic zone is far from the nearest
plate boundaries, which are in the middle of the Atlantic Ocean and in
the Caribbean Sea. The seismic zone is laced with known faults but numerous
smaller or deeply buried faults remain undetected. Even the known
faults are poorly located at earthquake depths. Accordingly, few, if any,
earthquakes in the seismic zone can be linked to named faults. It is difficult
to determine if a known fault is still active and could slip and cause an
earthquake. As in most other areas east of the Rockies, the best guide to
earthquake hazards in the seismic zone is the earthquakes themselves.

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